The modulated light energy of known electrooptical light modulating devices varies sinusoidally as a function of the amplitude of an applied voltage signal. Practically, only a portion of the sinusoidal curve is utilized for intensity modulation. However, the electrooptical characteristic of the light modulators is liable to vary as a function of the heat generated in the device during operation due to the passing light energy so that the operating curve is caused to shift and consequently the biasing point of the modulator drifts from the previous operating point. This results in a variation of the mean-value energy of the modulated light beam as well as in the degree of intensity modulation. To keep the degree of modulation constant, peak detectors may be used to detect the positive and negative peak values of the modulating signal. However, peak detection is difficult to achieve if the frequency of the modulation signal is high.